1. Field of the Invention
The present invention relates to an image formation apparatus, such as a copying machine, a printer, or a facsimile, that forms an image by electrophotography, electrostatography, iconography, magnetic recording method, or the like as well as to a driving control method for an image carrier and a computer program product that stores therein a computer program for executing the driving control method.
2. Description of the Related Art
As an image formation apparatus of this type, an image formation apparatus having a transfer nip is widely known. In the apparatus, the transfer nip is formed by an image carrier, such as a photosensitive element or an intermediate transfer member, and a transfer member, and a visible image formed on the image carrier is transferred onto a recording medium such as a sheet. In such an image formation apparatus, an impact that occurs when the recording medium enters the transfer nip momentarily causes, to the image carrier, a large load torque that causes a change in the surface moving velocity of the image carrier to result in image degradation.
Japanese Patent No. 3528342 and Japanese Patent Application Laid-open No. H2-53083 disclose the technology that can reduce the image degradation caused by an impact that occurs when a recording medium enters the transfer nip in the image formation apparatuses.
An image formation, apparatus disclosed in Japanese Patent No. 3528342 or that disclosed in Japanese Patent Application Laid-open No. H2-53083 includes a braking device that exerts a braking force on a driving source of an image carrier. The braking device exerts a braking force (driving load torque) corresponding to a load that is to be applied when a recording medium enters the transfer nip to the image carrier in advance before the recording medium enters a transfer nip. The image formation apparatus also measures a torque change on the image carrier that occurs when a recording medium enters the transfer nip in advance by an experiment, a computer simulation, or the like, and holds the thus-obtained torque variation profile (waveform of the torque variation). And then, by adjusting to the timing when the recording medium enters the transfer nip, the braking force provided by the braking device can be reduced or removed in accordance with the torque variation profile, thereby reducing the variation in the surface moving velocity of the image carrier.
However, there has been a problem in the conventional image formation apparatus that exerts a braking force (driving load torque) on the driving source in advance and reduces or removes the braking force when a recording medium is going to enter the transfer nip so as to suppress a variation in the surface moving velocity of the image carrier, as described below.
It has conventionally been considered that a torque variation profile of the image carrier occurring when a recording medium enters the transfer nip can be treated as essentially the same between different recording media so long as the recording media are similar to each other (e.g., the recording media having a same thickness). Therefore, as described above, the conventional image formation apparatus typically obtains by prior measurement for each type of the recording media, on which images are to be formed, a profile of a torque variation that occurs when the recording medium enters the transfer nip, and controls the braking device so as to cancel the torque variation based on the torque variation profile at a timing when the recording medium enters the transfer nip. Even with such control, a torque variation on the image carrier occurring when a recording medium enters the transfer nip can be cancelled with high accuracy in a case where the image formation is performed in the same condition as that used in the measurement of the torque variation profile and where the image formation is performed to the same kind of recording medium as that used in the measurement.
In practice, however, a type of a recording medium to be used depends on a user of the image formation apparatus. Furthermore, an environment or a condition, in which image formation is to be performed, also depends on the user of the image formation apparatus. For these reasons, in many cases, an actual torque variation profile occurring when the recording medium enters the transfer nip does not match the torque variation profile obtained by the prior measurement. In particular, the torque variation profile occurring when the recording medium enters the transfer nip can change due to various types of factors even when the recording media are identical to each other in thickness and material properties. Examples of the factors include a difference in humidity, a difference in degrees of skew or curl of the recording media in entering the transfer nip, a difference in the direction of fibers forming the recording media, and a difference in the lengths of the recording media in a recording-medium conveying direction. Accordingly, it is impracticable to prepare every torque variation profile that is anticipated to occur in an actual image formation process. Even if every torque variation profile that is anticipated to occur in the actual image formation process can be prepared, it is considerably difficult to select a profile optimum to each image formation process from the thus-prepared large number of torque variation profiles.
As described above, the conventional image formation apparatus that tries to cancel a torque variation in the driving source occurring when a recording medium enters the transfer nip on the basis of the torque variation profile prepared in advance cannot cope with all the actual torque variation profiles that vary from one image formation process to another depending on the various kinds of factors. Thus, in many cases, the conventional image formation apparatus is unable to cancel the torque variation of the image carrier with sufficient accuracy or to suppress a variation in the surface moving velocity of the image carrier.
Furthermore, for the conventional image formation apparatus, in order to cancel the torque variation of the image carrier by braking control based on the torque variation profile obtained in advance, it is necessary to cause a phase of the torque variation profile obtained by a prior measurement to coincide with an actual torque variation profile occurring when a recording medium enters the transfer nip. In order to achieve this coincidence, it is necessary to detect, with high accuracy, the timing when the recording medium actually enters the transfer nip. The timing when the recording medium actually enters the transfer nip varies from one image formation process to another even when the recording media are of a same type. Accordingly, the timing when the recording medium actually enters the transfer nip is typically determined by using a sensor. Meanwhile, it is difficult to directly detect the position of a leading-end of the recording medium that enters the transfer nip. Therefore, in general, detection is made with the sensor on the position of the leading-end of the recording medium a certain period of time before the recording medium enters the transfer nip. Accordingly, if a conveying velocity of the recording medium differs from a target conveying velocity, an error occurs in the timing, determined by the detection result with the sensor, for the recording medium to enter the transfer nip. A detection error of the sensor or an assembly error of the sensor also causes an error in the timing for the recording medium to enter the transfer nip according to the determination by the detection result with the sensor. Such an error makes it difficult to detect the timing when the recording medium enters the transfer nip with high accuracy. Therefore, in many cases, the conventional image formation apparatus, for which it is necessary to detect timing when a recording medium enters the transfer nip, fails to cancel the torque variation of the image carrier with sufficiently high accuracy, and hence cannot reduce the variation in the surface moving velocity of the image carrier.
A control method, which is typically employed in a conventional image formation apparatus, of obtaining a torque variation profile by a prior measurement and performing braking control based on the torque variation profile so as to cancel torque variation of an image carrier is primarily incapable of canceling torque variation of an image carrier caused by an impact whose occurrence is unpredictable or unable to be predicted with high accuracy. Even for an impact whose occurrence is predictable with high accuracy, if a profile of torque variation in the image carrier caused by the impact is irregular, the torque variation cannot be cancelled with high accuracy.
Furthermore, in the control method, that is typically adopted in the conventional image formation apparatuses, of canceling torque variation of an image carrier by braking control, it is necessary to exert a braking force (driving load torque) on the image carrier during when the impact is not applied to the image carrier. However, according to such a control method, it is necessary to apply an additional driving torque that corresponds to a driving load torque to be caused by an impact during when the impact is not applied to the image carrier. Accordingly, this control method, requiring a driving source of an image carrier to generate the additional driving torque, is disadvantageous in increasing power consumption.
This control method also requires that the magnitude of the braking force (driving load torque) to be exerted on the image carrier in advance be set to have a greater value than a driving load torque caused by a real impact. Accordingly, the control method is also disadvantageous in that it is necessary to determine, in advance, a driving load torque to be caused by a real impact.